Cathode-ray tube of the masked target variety



NOV. 10, 1953 w s m 2,659,026

CATHODE-RAY TUBE OF THE MASKED TARGET VARIETY Filed April 12, 195.1

Patented Nov. 10, 1953 TED STATES PATENT OFFICE CATHODE-RAY TUBE OF THE MASKED TARGET VARIETY David W. Epstein, Princeton, N. J., assignor to Radio Corporation of America, a corporation of Delaware This invention relates to improvements in color-kinescopes and other cathode-ray tubes of the so-called masked-target variety.

Masked-target color-kinescope are described in an article entitled General description of receivers for tri-color kinescopes, in the June 1950 edition of the RCA Review. They are described in greater detail in Alfred N. Goldsmiths application, Serial No. 548,239, filed August 5, 1944, which was refiled on July 19, 1947 as Serial No. 762,175 (now U. S. Patent No. 2,630,542) and in the copending applications of A. C. Schroeder, Serial No. 730,637, filed February 24, 1947 (now U. S. Patent No. 2,595,548), Hanna C. Moodey, Serial No. 166,416, filed June 6, 1950, and Russell R. Law, Serial No. 143,405, filed February 10, 1950.

The color-screen or target used in most color-kinescopes of the masked-target variety comprises an orderly array of small, closelyspaced, aluminized phosphor-dots arranged in triads (i. e. triangular groups) comprising a green-emitting dot, a red-emitting dot and a blue-emitting dot. In the interests of good optical resolution the screen should contain as many phosphor-dots as possible. The 100 square-inch screen used in present day tri-color kinescopes usually contains 200,000 triads, i. e. 600,000 separate dots in all. The particular phosphor-dot illuminated at any given moment is determined by the angle at which the scanning beam passes through the shadow mask and approaches the screen. Because of the large number of dots, the differences in the (three) angles of approach required to activate difierent dots in the same triad must be very small and, usually, are about 1 off a common axis in register with the center of any given aperture.

Several factors dictate the use of a shadow mask of the smallest possible thickness dimension. Among these factors are: (1) color dilution caused by stray secondary-electrons striking non-selected ones of the color-dots. It is known that most of the disturbing secondaries come from the boundary-walls of the apertures in the mask. Hence, by making the mask very thin, color-dilution is minimized by reason of the reduced number of secondary-electrons occasioned by the reduction in the area of their origin, (2) The mask must contain one aperture for each triad of phosphor dots. Thus, a mask designed to serve a screen having 200,000 triads or groups of dots must contain 200,000 apertures. The most practical way of making so many apertures in a metal plate is by the use of photo-engraving or similar (etching) methods which lend themselves more readily to thin than to thick metal. (3) The thinner the mask, the lower the tensile forces required to maintain it taut. This simplifies the problem of supporting the screen assembly.

With the foregoing factors in mind it might be said that the ideal shadow-mask would be one of infinite thinness. Since this ideal is impossible of achievement, the practice has been to make the mask as thin as may be practical. The practical limiting factor, of course, is the ability of thin metal to resist warping when it is subjected to the heat resulting from impact of an electron beam (or beams) of the intensity or velocity required to achieve a light-image of the desired brightness. If the mask does warp, its apertures become mis-aligned with the individual centerpoints of the triads of phosphor-dots and the masking efiect is lost.

The use of a supporting frame of special construction (see co-pending application of H. B. Law, Serial No. 158,901); now U. S. Patent No. 2,625,734 permits the use of a metal (superniekel) mask only 0.004 inch. However, even here the thickness dimension of the mask imposes an undesirably low upper limit upon the output of light from the tube by limiting the intensity of the beam (or beams) to a value low enough to avoid warping.

Accordingly, the principal object of this in vention is to provide a method of and means for overcoming the foregoing and other less apparent objections to present day cathode-ray tubes of the masked target variety.

Another and specific object of the present invention is to provide an improved masked-screen assembly for use in cathode-ray tubes, and one characterized by its freedom from warping when subjected to prolonged bombardment by electrons of the velocity required to achieve a light image of a satisfactorily great brilliance or brightness.

Another and important object of the invention is to provide an improved color-kinescope, and one characterized by the brightness of its colors, its freedom from color-dilution and by its long useful life.

The foregoing and other objects are achieved in accordance with the method of the invention by intercepting the scanning beam or beams in a plane intermediate the beam source and the apertured mask during at least a fraction of that portion of the scanning movement whereat. the beam is traversing the space between adjacent ones of the spaced-apart color-areas 011 the screen. In the best embodiment of the invention, herein illustrated, the interception of the beam is accomplished by the use of an auxiliary metal mask of a plate-like construction similar to that of the shadow mask, but having somewhat larger apertures so that the impact of the beam is divided between the two masks in order to prevent excessive heating of either mask.

The invention is described in greater detail in connection with the accompanying drawing, wherein:

Fig, l is a longitudinal sectional view of a three-gun tri-color kinescope embodying the invention;

Fig; 2 is a fragmentary view in perspective, on an enlarged scale, of the improved target or screen assembly of the tube of Fig. 1; the drawing being marked to show the axes of the three electron-beams; and,

Fig. 3 is an enlarged fragmentary sectional view of the target or screen assembly, showing the manner in which the impact and hence the heat of the beams is divided between the shadowmask and the auxiliary mask.

The color-kinescope shown Fig. 1, comprises an evacuated envelope I having a main chamber in the form of a frustum 3 which terminates in a window 5 through which a transparent or translucent viewing screen I is visible. The other or small end of the irustum terminates in a neck portion 9 which, in the instant case, contains three electron-guns H, l3 and is arranged 120 apart (A-rashion) about and parallel to the long axis of the tube, as claimed in Schroeder 2,595,548.

The screen 5, here illustrated, is of the dot-like variety claimed by Goldsmith 2,630,542. It is provided on its rear or target surface with a multiplicity (say, 600,000) phosphor dots, R (red), B (blue), G (green) of different color emissive characteristics. The dots are arranged in a hexagonal pattern, in triads or groups-ofthree. (Thus, each dot is surrounded by six other dots.) Also as in the Goldsmith disclosure, an apertured shadow-mask I! is disposed in front of the target surface of the screen 1. The mask Ii comprises a thin metal plate containing 200,000 holes, or one hole for each of the tricolor dot-groups.

The other elements in the main chamber i of the tube are: The heat-absorbing auxiliary mask 19 (later described) of the present invention, and a conventional, hollow, conical magnetic shield 2| surrounding the long axis of the tube to the rear of the auxiliary mask Hi.

There are two magnetic yokes 23 and 25 on the neck of the tube. The yoke or coil 23, which is disposed adjacent to the common anode 2 of the battery of guns it, i3, i5 operates to converge the axes of the emerging parallel beams r, b and g upon a common point p (Fig. 2) in the plane of the shadow mask 12. In the embodiment shown in Fig. l, the conductive coating 28 on the inside of the neck of the tube is at the same potential as the electrode 21, and serves as an accelerating electrode. The converging coil 23 can be eliminated by operating the coating 28 positive with respect to the electrode 2'7, thereby creating a converging electron lens. As shown in both Figs. 2 and 3, the paths of the beams diverge at the point p and terminate on separate ones of the phosphor dots, R, B and G, respectively. The other yoke 25 will be understood to comprise a pair of deflecting coils for imparting a conventional (horizontal and vertical) scan- 4 ning movement to the beams in their passage to the screen or target assembly.

Each hole 29 in the shadow mask I1 is so registered with its associated dot group that the difference in the angle of approach of the three scanning beams r, b and 9 determines the color. Thus, three color signals applied to the three guns H, i3, i5 produce independent pictures in the three primary colors, the pictures appearing to the eye to be superimposed because of the close spacing of the very small phosphor dots. The

-shadow mask H, as used in present-day colorkinescopes stops about of the beams power. When, for example, the mask I! is made of super-nickel, about 0.004" thick, and has about square inches of target area, it shows a tendency to warp when the beam current exceeds say, 400 micro-amperes. At 18 kilovolts the beam power is about 7.2 watts, hence, buckling may be said to start when the power density exceeds about:

or .06 watt per square inch. This undesirably low limit upon the beam-power in tubes of the masked-target variety no longer obtains when, in accordance with the method of the invention, the electrons are prevented from strikin the shadow mask l7 during at least a fraction of that portion of the scanning movement whereat the beams are traversing the space between adjacent ones of the holes 29 in the shadow mask ii. The interception of the beams during said portion of the scanning movement is best accomplished by the use of .an auxiliary mask 15 similar to the one shown in the drawing.

The manner in which the auxiliary mask l9 serves to permit the use of higher beam power (and a resultant increase in the brilliance of the image) without sacrificing the advantages incident to the use of a very thin shadow mask will be evident upon an inspection of Fig. 3. Here the red beam 7 and the green beam 9 are shown in cross-section (instead of being indicated merely by their central axes, as in Figs. 1 and 2) and are shown approaching the target assembly at the diii'erent angles required respectively to activate the red (R) and the green (G) phosphordots on the screen I. It will be observed that while the diameter of the apertures 31 in the auxiliary mask I9 is approximately twice the diameter of the apertures 29 in the shadow mask II, the diameter of the beam (1" or g) is larger than that of the first mentioned aperture 31. Hence, whether the beam or beams are in register with the common center line of the apertures 3| and 29 or are at some point in the scanning movement, the impact of the electrons is divided between the two masks l9 and I1. Thus, the addition of the auxiliary mask 19 to the target assembly of an otherwise conventional maskedtarget tube permits the use of about twice the beam-power, provided of course that the spacing between the masks, and the relative size of the masks apertures are such that the power to be dissipated (in the form of heat) is divided equally between the two masks.

The auxiliary mask It may be made to accept more or less than 50% of the beams impact, if desired, simply by a judicious selection of the size of its apertures 3i and its spacing with respect to the shadow .mask IL'I. However, the apertures BE in the auxiliary mask l9 should not be made so small that their boundaries cast their shadow directly upon the screen.

As a specific example of mask-spacing and aperture size let us assume the usual case wherein the phosphor dots R, B and G on the screen i are centered at points 120 apartabout a common axis and are each approximately 0.0137" in diameter. In this case the shadow mask II, which may be made of super-nickel about 0.004" thick should preferably be mounted approximately 0.5" in front of the target surface of the screen 1 with its apertures properly aligned with the axes of the individual triads. The diameter of the apertures 29 in the shadow mask I! should be about 0.010", i. e. slightly smaller than the diameter of the phosphor dots.

Like the shadow mask [1, the auxiliary mask l9 may be constituted of super-nickel, though it may be made of copper or other metal or alloy having good heat-conducting properties. It may be of thicker construction than the shadow mask (e. g. 0.010") and may be spaced from practically 0." to about 0.10" from the latter.

In applying the invention to a screen-assembly of the type (exemplified by German Patent 736,575) wherein the electron-sensitive areas on the target surface of the screen are in the form of lines (instead of dots) the apertures in the shadow-mask and in the auxiliary or "heat absorbing mask should be of the same (linelike) contour. It need scarcely be pointed out that the invention is not limited in its useful application to tubes employing any particular number of electron-guns or any particular type of scanning system. Reference may be had to the prior art listed in the second paragraph of this specification for alternative forms of beamsources and scanning systems.

What is claimed is:

1. Method of preventing warping of the thin apertured-metal masking-plate in a cathode-ray tube of the masked-target variety due to thermal effects caused by impact of a scanning beam upon said thin metal plate, said method comprising, intercepting a portion of said beam in a plane intermediate the beam-source and said plate during a fraction of that part of the scanning movement whereat the beam is traversing the space between the apertures in said plate.

2. A cathode-ray tube comprising, a beamsource of electrons, a screen mounted in a position to be scanned by electrons from said source, a shadow-mask in the form of a thin metal plate mounted in the path of said electrons and containing apertures through which said electrons pass in the transit to said screen, and an auxiliary apertured-plate mounted between said beam-source and said shadow mask with the apertures in said plates aligned with each other, the apertures in said auxiliary plate being of a diameter larger than that of the apertures in said shadow mask, whereby said beam in its transit to said screen impinges partly upon both of said plates with a consequent reduction in the quantity of heat which would be applied in the absence of said auxiliary plate, to said shadowmask by the impact of said beam thereon.

3. The invention as set forth in claim 2 and wherein said screen comprises a multiplicity of discrete, dot-like electron-sensitive, light-emissive areas each of a diameter smaller than that of the apertures in said auxiliary plate and slightly larger than the diameter of the apertures in said shadow mask.

4. A screen assembly for cathode-ray tubes of the masked-target variety, said assembly comprising a screen having a target surface, an apertured shadow-mask in the form of a thin metal plate mounted adjacent to the target surface of said screen and an auxiliary apertured plate mounted adjacent to the surface of said shadow mask which lies remote from said screen with the apertures in both plates aligned with each other.

5. The invention as set forth in claim 4 and wherein the apertures in said shadow mask are smaller than the apertures in said auxiliary plate.

6. The invention as set forth in claim 4 and wherein the target surface of said screen comprises a plurality of discrete electron-sensitive dot-like areas each of a diameter slightly smaller than the diameter of the apertures in said shadow mask.

7. The invention as set forth in claim 6 and wherein said discrete electron-sensitive areas are disposed in groups of three about the common axes of symmetry of the apertures in said plates.

DAVID W. EPSTEIN.

References Cited in the file of this patent UNITED STATES PATENTS Number Name Date 2,137,888 Fuller Nov. 22, 1938 2,315,367 Epstein Mar. 30, 1943 2,463,535 I-Iecht Mar. 8, 1949 FOREIGN PATENTS Number Country Date 855,065 France Mar. 31, 1941 

